With the exception of entomologists or herpetologists, few people likely enjoy whiling their days away with insects or other creepy crawlies that populate many horse farms. Although insects such as mosquitoes and ticks, which we described in June and July, can transmit diseases, they don't tend to draw quite as much attention as venomous spiders and snakes. These critters can strike fear in even the most stoic horse owner.

But fear not: In the final leg of this series we'll probe (gently!) the exciting worlds of snakes and spiders to help owners avoid clashing with these creepy crawlies as well. Our sources also will provide information about how to handle snake or spider bites should your horse have an unfortunate run-in with one of these critters.

Slithering Snakes

Snakebites are a very real concern for owners living in regions where venomous snakes reside, particularly in Texas and Florida. Approximately 20 different venomous snake species exist throughout the United States, and at least one of these species is found in every state except Alaska and Hawaii. Every year, venomous snakes bite an estimated 8,000 U.S. residents; fortunately, death due to snakebite is extremely rare. Only about five to 12 humans die following snakebites every year, and rattlesnakes and copperheads are the common culprits.

"One study published in the April 2007 edition of the Veterinary Clinics of North America estimates that several hundred horses are bitten by snakes annually," says Tracy E. Norman, VMD, Dipl. ACVIM, of the Department of Large Animal Clinical Sciences at Texas A&M University. "Because most cases of snakebites are treated by primary care veterinarians, it is hard to know exactly how many horses are bitten. I usually only see snakebites (referred to the university clinic) when complications ensue, which is really quite rare."

Angela Pelzel, DVM, a Western region epidemiologist with the USDA's Animal and Plant Health Inspection Service (APHIS), adds, "I used to see about two or three venomous snakebites per year as a primary care practitioner in Central Texas. I've never had any horses in practice die from venomous snakebites, but all of my snakebite victims were adult horses in otherwise good health whose owners correctly sought veterinary care right away."

Venom, which helps snakes digest their prey, is dangerous because of the different "fractions" it contains. These chemicals include proteins, amino acids, enzymes, fats, metal ions, and myriad unidentified ingredients. Venom varies from snake to snake, and some venoms are more toxic than others. For example, a Mojave rattlesnake's venom is almost 50 times more deadly than a copperhead's (based on studies in mice). The venom toxins can spread rapidly through the body, causing extensive ¬tissue destruction, blood clotting abnormalities, heart damage, and nerve damage.

The two most important rules for dealing with snakes are to remain calm and keep your distance; even a dead snake can envenomate (for instance, a snake's detached head can have an immediate reflex to bite). Fortunately, most live snakes prefer to avoid confrontation and will slither away quickly rather than chasing you or your horse down. Further, approximately 20% of rattlesnake bites are "dry," meaning they inject no venom when they bite; this can occur because snakes control the amount of venom injected into a victim. Typically, though, snakes inject larger amounts of venom into larger prey.

Identifying and Treating Snakebites

Because horses are inquisitive, they spend much of their waking time grazing, and they have poor close-up vision, guarding them from snakebites can be difficult. And identifying a snakebite can be just as challenging.

"Often, owners don't realize their horse has been bitten by a snake (particularly if there is only localized pain and swelling) and therefore don't call their veterinarian," notes Norman. "This means that some life-threatening complications can occur, such as heart disease, laminitis, and liver failure, which often have a delayed onset and are frequently overlooked until the disease is advanced."

If you suspect a snake has bitten your horse, based on extensive unexplained swelling, call your veterinarian as soon as possible. While waiting for his or her arrival, remain calm and minimize your horse's movement; the latter can slow venom spread through the body.

"Do not touch the wound," Norman emphasizes. "Skin contact with the venom can put a person at risk." Similarly, do not attempt to capture or touch the snake, even if it appears dead. If you are snake savvy, try to identify the species to help your veterinarian treat the horse appropriately.

Horses usually sustain bites on the face or legs and develop considerable swelling at the bite site within an hour if envenomation occurred (i.e., a wet bite. Dry bites are often not noticeable unless an owner witnesses a bite). Swelling inside the nostrils can become so severe that it impedes breathing and, because horses are obligatory nasal breathers, this presents a real danger.

"If the horse's nose is swelling severely and there is fear that the airway will be blocked before a vet can get there, placing a rigid tube in the nose (e.g., a large-bore syringe case with the end cut off) may keep the horse's airway open," Norman advises.

Similarly, Pelzel recommends horse owners living in venomous snake country keep two sections of 12-16-inch-long garden hose in their emergency equine first-aid kit; owners or veterinarians can immediately place these hose sections in both nostrils to help keep the snake-bitten horse's airway open. She notes that owners can tie the hoses to a loose-fitting halter (that will accomodate facial swelling) to keep them in place, especially once the swelling begins to subside.

Generally, the veterinarian prescribes a round of anti-inflammatory drugs to target pain and swelling, and the horse should wear the hoses for a few days until the swelling goes down.

Conditions that mimic spider bites

According to Richard Vetter, MS, staff research associate at the University of California, Riverside, Department of Entomology, and Tracy E. Norman, VMD, Dipl. ACVIM, of Texas A&M University's Department of Large Animal Clinical Sciences, an extensive list of conditions can cause wounds that mimic spider bites. Here are some to consider in horses:

Bacterial skin infection (with Staphylococcus or Streptococcus spp);

Pigeon fever;

Lymphangiectasia (pathologic dilation of lymph vessels);

Lyme disease;

Fungal infection;

Chemical burn;

Squamous cell carcinoma; and

Localized vasculitis (inflammation of the blood vessel walls).

--Stacey Oke, DVM, MSc

Veterinarians often treat snakebites with anti-inflammatory drugs to help control the pain and swelling; a tetanus booster (recommended for all horses sustaining a wound or undergoing surgery); and broad-spectrum antibiotics to prevent secondary bacterial infections. Depending on the degree of envenomation or if complications occur, veterinarians might perform more aggressive therapy. For example, horses systemically ill due to snakebite-associated complications could require a tracheotomy (creating an opening in the trachea, or windpipe, to permit breathing), a plasma or blood transfusion (to help with blood clotting), and intravenous fluids (to maintain hydration and for cardiovascular support). Corticosteroids, antihistamines, and even hydrotherapy should be used with caution (if at all), advises author Christine Rees, DVM, Dipl. ACVD in the Textbook of Equine Internal Medicine, 3rd ed (2010).

Although veterinarians can treat most horses successfully, reports indicate 10-30% of horses bitten by snakes do not survive. "Most of the horses that don't make it either die or are euthanized because of the secondary complications I mentioned earlier, not the acute envenomation," Norman explains.

Contrary to popular belief, treating the horse with antivenin is not always the answer. "At Texas A&M, the availability of the antivenin has been inconsistent in the past and costs approximately $600 to $3,000," relays Norman. "The antivenin is usually only used when major envenomation is suspected. Still, there is little information in the published literature about the overall effectiveness of the antivenin, and there is no data indicating how quickly the horse must be treated."

Stealthy Spiders

There are lots of urban myths involving spiders, such as the cactus filled with baby tarantulas and, of course, that bananas imported from certain countries harbor deadly spiders. Perhaps one of the most famous spiders (infamous, even) is the brown recluse, which is prevalent from the southern midwest United States south to the Gulf of Mexico. Rumor has it a single bite can kill almost instantly. But wait: It turns out the brown recluse and many of its relatives might have been wrongfully convicted of crimes they haven't committed.

Spiders are often the "fall guys" for other, more common causes of skin lesions (see sidebar), says Richard S. Vetter, MS, a staff research associate who studies spiders in the University of California, Riverside, Department of Entomology. "Almost all spiders in the U.S. are 'venomous,' " he explains, "but very few have venom that is sufficiently toxic to humans or animals such as horses, which means significant enough that you notice (the bite) and try to remedy or at least watch."

Most spider bites typically only cause transient pain and swelling. Equine victims could sport obvious teeth marks if bitten on a hairless part of their body (e.g., muzzle), but the vast majority of spider bites are innocuous, even if the spider is known to be toxic.

Vetter says the only groups of spiders that are "bona fide toxic to humans" are widow and recluse spiders. He adds that there are only three widow species in the United States that share the same geographical niche as humans, and only a few species of recluses that are any concern to humans.

"Most households, such as the millions of people in the midwestern U.S. that live with brown recluses, never experience a bite," says Vetter. "Moreover, 90% of all brown recluse bites heal without severe scarring, and many brown recluse bites result in a small, red mark (on human skin) that heals by itself. Most bites are virtually benign so they shouldn't be expected to cause problems in horses all the time, either."

That said, arachnologists do recommend "spider proofing" house and barn areas to minimize bite risks to both humans and horses. "Don't leave things lying around," Vetter advises. "Recluse spiders love clutter. Inspecting horse blankets and saddles and rolling and crushing fly masks before putting them on the horses is good for (eliminating) either widows or recluses because they will bite when pressed against flesh. That said, I would be surprised if a spider with tiny fangs could fight its way through all the horse hair (even areas with finer hair, such as the face) to lay a bite on flesh or survive being squashed by the weight of a saddle."

Being safe is better than being sorry, but Vetter re-emphasizes that humans, horses, and other mammals can live cohesively with spiders without incident.

"In endemic recluse areas they can be very abundant," he relays. "Some colleagues of mine removed 1,100 recluses from a barn in three nights, and I collected 40 in a Missouri barn in about 30 minutes. A family in Kansas collected over 2,000 recluses in their house in six months, have horses and a barn full of recluses, and no horse bites were ever found."

But like spotting snakebites, recognizing a spider bite can be challenging. "Horses can have anything from localized to generalized swelling, especially on a limb and can have low-grade fever (102.5°F) and (appear as if they) feel lousy," Norman says. "Spider bites are usually self-limiting (resolve on their own), but may require more treatment if they result in local tissue necrosis (death) and infection. It is always best to call your veterinarian to examine any sick horse with a swollen body part."

Norman also notes, "Widow spider bites can be very painful and cause swelling, but would not cause the tissue necrosis that is seen with brown recluse spider bites. Brown recluses have phospholipase D in their venom that is classified as a necrotoxin and causes local tissue-destruction."

Both spiders can potentially bite horses, but the brown recluse does more damage.

No Cause for Pathophobia

Pelzel maintains that disease transmission via snakes and spiders--unlike biting insects--is not a concern. (It's important to note that ticks are members of the arachnid family, but generally aren't considered spiders.) "To my knowledge, snakes and spiders are not known to transmit disease or spread disease between horses," she says. "Snakes do inject, but they don't appear to have the capability to suck blood back into the fang mechanism. In fact, given the copious flow of venom usually expelled, the venom would actually flush out the fangs and really prevent any blood uptake or even much blood contamination."

Pelzel also notes that, in her experience, venomous snakes don't usually double strike. Rather, they typically strike at their offender once and take off.

So while the chances of death due to snake or spider bite are more than 23 million to 1, be aware of snakes and spiders and keep an eye out for bites if you live in an area where these animals thrive. Always call your veterinarian promptly with any medical concerns such as malaise, swelling, and fever.

About the Author

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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